1. Field of the Invention
The present invention relates to a self-driven welding robot for welding rectangular fillet weld joints of a product in the field of heavy industries and, more particularly to a self-driven lattice welding robot to which is mounted a welding gun having an arc sensor function.
2. Description of Related Arts
Lattice welding robots which automatically perform rectangular fillet welding of a lattice block, of which the base material is of a lattice shape, by using a high-speed rotating arc welding torch of the arc sensor type, have been disclosed in Japanese Unexamined Patent Publications No. 64-15286, No. 64-15287, and No. 4-319072. All of these publications, having a high-speed rotating arc welding torch, automatically perform rectangular fillet welding of a lattice block while carrying out tracer control of a weld line by a known high-speed rotating arc welding method of the arc sensor type. However, because the employed welding carriage is of a self-driven type, the travel direction of the carriage is not always in parallel with the weld line due to disturbances such as the state of a cable and the state of the work. Therefore, although the tracing control is carried out precisely by the arc sensor, the welding speed at the weld line portion changes, or the range in which the torch aiming position is corrected by the stroke limit of the arc sensor is limited.
For this reason, it is required for the welding carriage to trace the travel direction roughly by steering the carriage so that the travel direction is nearly parallel with the weld line.
For the conventional welding robots, a rough guidance control for carriage travel has been carried out by the following methods:
(1) Guide roller system shown in Japanese Unexamined Patent Publication No. 64-15286:
The robot disclosed in this publication, which is provided with two tracing guide rollers at the side of the welding carriage, performs a rough tracing of the weld line while bringing the guide rollers into contact with a vertical plate surface of a lattice block.
This system, however, has disadvantages such as (1) it is influenced by the state of the vertical plate surface, and (2) the guide rollers hinder the turning of the welding robot.
(2) Non-contact distance sensor system shown in Japanese Unexamined Patent Publication No. 4-319072:
This system uses two non-contact distance sensors in place of the aforementioned guide rollers. The two distance sensors detect the distance between the welding carriage and a vertical plate, and the speeds of the inside and outside wheels of the carriage are controlled so that the two detected distances are equal.
However, this system using, for example, a laser sensor, is very vulnerable to fumes and is expensive.
(3) Arc sensor system shown in Japanese Unexamined Patent Publication No. 64-15287:
This system provides rough tracing of a weld line, so that the distance between the welding torch and a vertical plate is nearly constant, on the basis of the information about the horizontal (X-axis)slide position of the welding torch, controlled by an arc sensor.
However, the distance information of only one point does not tell the inclination of the welding carriage, so that travel control of the carriage is difficult to carry out.